Do we live in a parallel universe?
Infinite Earths and alternate realities: does this controversial theory have any scientific basis?
It’s an understatement to say that the multiverse theory is one of the most controversial theories in science. In fact, merely putting this in the Space section of the magazine, and not a newly created Theology section, would ruffle a few astrophysicists’ feathers. But why is this the case, and is there any basis for suggesting we live in a multiverse?
The origins of the multiverse theory are a grey area. Some, like David Deutsch in his book The Beginning of Infinity, point to Erwin Schrödinger and his famous equation. This broadly introduced the idea of quantum mechanics, in which a particle can be in two states at once, in the first half of the 20th century. It would be many years until the broader implications of the theory were given serious thought, though. You’re probably more familiar with the multiverse theory in different terms – parallel universes – so let’s begin there. At its core, the multiverse theory suggests that our universe is not alone, but perhaps one of many in some form or another. Just as we discovered Earth was one of many planets, and that the Milky Way was one of many galaxies, some scientists think the same could be said of the universe. As of yet, we have no direct evidence for multiverses (and even that prospect is contentious, which we’ll come on to later). But our best indirect evidence for its existence is a peculiar one. It stems from how exact certain mathematical constants in the universe are. The cosmological constant, for example, is a value for the energy density of the vacuum of space. Its existence explains how the universe is expanding at an ever-increasing speed, something first discovered in 1998.
But the cosmological constant is 120 orders of magnitude smaller (that is, ten to the power of minus 120) than theory predicts it should be. Thus, even a small change in its value would have rendered our universe a mess of nothingness after the Big Bang. So, too, for the values of dark energy. How were these mathematical constants so finely created? “If [dark energy] had been any bigger, there would have been enough repulsion from it to overwhelm the gravity that drew the galaxies together, drew the stars together, and drew Earth together,” Stanford physicist Leonard Susskind told Discover Magazine in 2008. “It’s one of the greatest mysteries in physics. All we know is that if it were much bigger we wouldn’t be here to ask about it.”
Working on the pretence that this is true, what form would these other universes take? That’s the tricky part.
The multiverse theory has an answer, though. It suggests that in our universe, the cosmological constant is exactly the right value for everything as we know it to exist. But there are an infinite number of other universes, where it is ever so slightly different. Working on the pretence that this is true, what form would these other universes take? That’s the tricky part. There are a large number of theories, from Max Tegmark’s four levels of classification (explained later), to M-theory (which encompasses string theory), to cyclic theories, where the universe is in an infinite number of cycles between Big Bangs and Big Crunches. Tegmark’s four levels encompass the broader multiverse theories. The Massachusetts Institute of Technology professor suggested them in 2003, presenting them as a way to classify ideas for the multiverse. “Parallel universes are not a theory, but a prediction of certain theories,” he said in his 2014 book Our Mathematical Universe. The first level deals with the observable universe, which is the extent to which we can see in the universe. Owing to the finite speed of light, we are only able to see as far as light has been able to travel to us since the Big Bang, 13.8 billion years ago. Due to the expansion of the universe, though, we are able to see light that is now more than 42 billion light years from us, which we call the observable universe. But we cannot see beyond this; what is there, we just don’t know.
Tegmark’s first multiverse level suggests that there is no end. Instead, the universe just keeps going and going, infinitely. If true, this would create an infinite number of instances for everything to occur. So, at some astronomical distance away from us, we would find an Earth exactly the same as ours, and you would find yourself sitting there reading this very article. The second level is similar to the first, but proposes that while the whole multiverse is expanding, there are regions within it that expand at different rates, forming bubbles of self-confined space – in other words, bubble universes. Our universe would be one bubble, with an untold number of other bubbles beyond, each with their own laws of physics. In 2015, a later, widely discredited theory suggested our bubble universe had actually ‘bumped’ into one another, producing a noticeable glow in the far reaches of space. In the third level, things start to get a little bit strange. Like the first, it suggests that the laws of physics are the same everywhere, but rather than different universes being separated by distance, as in the second level, they are in fact separated by time. The laws of quantum mechanics, as mentioned earlier, allow for a large number of uncertainties and possible futures (for example, whether Schrödinger’s famous ‘cat in the box’ is dead or alive). In this level, all of these possibilities would play out. Every single eventuality would occur, and each time, a new universe would be created along with it. For us as observers, though, we only see one universe – our own.
Our universe would be one bubble, with an untold number of other bubbles beyond, each with their own laws of physics.
The fourth and final level, the mathematical multiverse, is fairly difficult to comprehend. It is Tegmark’s own theory, presented in Our Mathematical Universe. It essentially implies that the universe is composed entirely of mathematics, and we are merely constructs within that. But the book and theory have come under some heavy criticism. One of the main arguments against the multiverse theory, though, is that it fails one of the very cornerstones of science itself: falsifiability. This is the ultimate test for any scientific theory, namely that it can be proven wrong. For example, if you put forward the theory that every animal on Earth had four legs, someone else could refute that theory by finding an animal with more or less than four. No multiverse theory is currently falsifiable. We simply don’t have the means to disprove some of the claims being made. We will never be able to journey beyond the observable universe, and thus could never disprove the notion that there are other parallel bubble universes out there, or an infinite universe.
As such, many argue that the multiverse theory should not be treated as a theory at all. It should be condemned to the pseudoscience bin. “The trouble is that no possible astronomical observations can ever see those other universes,” said cosmologist George Ellis in an article published in Scientific American in 2011. “The arguments are indirect at best. And even if the multiverse exists, it leaves the deep mysteries of nature unexplained.” Of course, falsifiability itself has its detractors. Other more widely accepted theories, such as the existence of dark matter or dark energy, may not be falsifiable. Should we also consign those to the scrapheap? It’s fair to say that this is a topic that draws heated debate in the scientific community. And even aside from falsifiability, we run into a problem. Not only can we not disprove multiverse theories, but we can’t currently prove them either. We have no way of jumping to another universe, or even observing one. How are we supposed to sift through the myriad of claims being made when there is no direct evidence available?
Did you know: One bizarre multiverse theory suggests, in an infinite multiverse, we could be a simulation made by aliens
The idea of a multiverse is undoubtedly an intriguing one. It has inspired a huge range of science fiction, and has garnered support from some of the most prominent physicists today. “It would not be beyond the realms of possibility that somewhere outside of our own universe lies another different universe,” Professor Stephen Hawking said in 2015. But it remains divisive, and will do so for the foreseeable future. For now, it remains a fringe theory in some corners. And perhaps in an issue of How It Works in an alternate universe, it is indeed confined to the Theology section.
This article first appeared in How It Works issue 88 written by Johnny O’Callaghan.
Could we live in another universe if the speed of light was different?
Universe is expanding quicker than previously thought